Field of the Invention
The present invention relates to color correction systems for video signals, and in particular, to color correction systems for film-to-video conversion systems.
Description of the Related Art
Color correction systems for selectively correcting electronic signals representing optical images are well known in the art. Various forms of color correctors are used in many sophisticated video systems. In particular, sophisticated color correctors are used in film-to-video conversion systems, such as telecines. Particularly in the case of telecines, color correction is needed to overcome color inaccuracies or nonlinearities introduced due to the nature of film and by the electronic scanning of the optical film images to produce the corresponding video signals.
Color correctors in such systems have typically required levels of skill beyond the capabilities of many, if not most, operators or editors. Indeed, color correction is often viewed as more art than science. With the growing use of video systems and the corresponding growth in need for editors for such systems, color correctors which require less skill, while providing equivalent if not better color correction, are needed.
Two aspects of color correction in particular have up to now required a great deal of skill, experience and insight to effectively operate conventional color correctors and provide the desired color correction. The first aspect, and perhaps the more difficult, involves correcting for film characteristics. Such film characteristics include crosstalk between the dyes in the multiple emulsion layers of color film, film speed, gamma, and minimum and maximum film dye densities. Further, film characteristics are generally nonlinear in nature, and are therefore very difficult to compensate, or correct.
A second difficult aspect of color correction involves video signal characteristics. Such characteristics include hue (or tint), saturation and intensity (or value). While perhaps more straightforward to compensate, or correct, than the aforementioned film characteristics, they are nonetheless nontrivial. Further, a video signal typically must be converted in accordance with a color standard which is dependent upon the video display device (e.g. cathode ray tube) and the video signal standard (e.g. NTSC or PAL) being used.
A major difficulty facing editors, or "colorists," is that most color correction circuitry in conventional color correction systems is analog, and therefore not very precise or flexible. If adjustments are needed to accommodate one particular parameter being corrected, most if not all other parameters are typically also affected to varying degrees. Some newer, more sophisticated color correction systems, such as special effects machines, do use some digital designs. However, these machines are generally video only, i.e. only video signals are processed, and therefore only the simpler corrections for video signal characteristics are provided. In other words, the more difficult corrections for film characteristics are not available. Thus, the digital color correction systems for video signal characteristics can be of minimal value in a film-to-video conversion system.
An additional related shortcoming to conventional color correction systems involves the retention of any color correction signals or data generated. Some systems have provided for digital storage of color correction data. However, this storage of color correction data is remote from the video signal itself, and related thereto only by some form of video signal identification data, such as timing information or frame number. Conventional color correctors have not provided for somehow embedding the color correction data within the video signal itself. Instead, they simply color correct the signal, thereby losing both the original uncorrected signal information and the color correction information or data.